TR-144

TECHNICAL REPORT

A STUDY OF AEROMAGNETIC COMPONENT DATA PLANTAGENET BANK

G.A. YOUNG and A. L. KONTIS

Marine Surveys Division Geomagnetics Branch

JANUARY 1964

U. S. NAVAL OCEANOGRAPHIC OFFICE WASHINGTON, D. C. 20390 Price 30 cents

AgBEomloR A) Col

Magnetic elements observed with a Vector Airborne Magne- tometer, under Loran-C control, are used to calculate the directional components for the anomaly associated with Plantagenet Bank. Also determined are the total intensity, inclination, and declination of the anomalous magnetic field. A theoretical source body is presented as a possible solution to the Plantagenet Bank anomaly.

FOREWORD

Although many oceanic crustal features are known to have magnetic anomalies associated with them, the description of these anomalies by airborne methods has been limited by a lack of navigational control. Project MAGNET aircraft have now acquired a positioning capability in areas of Loran-C coverage which is commensurate with the accuracy of the Vector Airborne Magnetometer.

This report contains the results of the first aeromagnetic component

survey using Loran-C for navigational control.

IN DENYS W OLL mikal, U. S. Navy

Rear Commdnder ?

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CONTENTS

Ih, JENBULOVDLUKC UE, 5 6 Gg 6 5 6 LES) DESCUSSION OR DATAS 3 3 505 3s; IIL. ESTIMATES OF SOURCE BODY ....

INOININEINGES 6 6.6 o 50 6 6 0-c

Io

13}.

FIGURES

SUAVE? ANE 6 Go OG to! loo 96 6 6 j6) 0) 6 ooo b 96 Bermudiay RASEM c) rrmnss 1s ie is) ish Oto Memes Weel arog alRtne wren Be coun nats Total Intensity (F) Contour Chart - Plantagenet Bank Inclination (1) Contour Chart - Plantagenet Bank . . Declination (D) Contour Chart - Plantagenet Bank . Anomalous X Contour Chart - Plantagenet Bank . Anomalous Y Contour Chart - Plantagenet Bank... .

Anomalous Z Contour Chart - Plantagenet Bank . .

Anomalous F Contour Chart - Plantagenet Bank .... Anomalous I Contour Chart - Plantagenet Bank... . Anomalous D Contour Chart - Plantagenet Bank... . Model Calculation of Source Body - Run No. 1... .

Model Calculation of Source Body - Run No. 2...

16

17

I. INTRODUCTION

Using Vector Airborne Magnetometers (Schonstedt and Irons, 1955), extensive aeromagnetic surveys of the magnitude and direction of the earth's magnetic field have been carried out by the Oceanographic Office since 1953. The primary purpose of this work has been to chart the earth's main magnetic field, and as such the surveys have been conducted at relatively high altitudes (6,000 - 20,000 feet), at track spacings of 200 miles, and the reduction of data has been limited to measurements at intervals of 15 - 20 miles along each track lime’.

Other requirements have necessitated fine grain, low level air- borne surveys of specific ocean areas, from which detailed total intensity information has been used to depict the magnetic field. The processing and analysis of detailed vector data over oceanic features have heretofore not been attempted. Using aeromagnetic survey data collected over Plantagenet Bank, this report describes a method for calculating the direction and magnitude of the anomalous field. In addition, a computer technique is applied to estimate the shape,

location, and magnetic properties of the source body.

IL. DISCUSSION OF DATA

The test data used for calculating the anomalous field components were provided from an aeromagnetic survey over Plantagenet Bank (Fig. 1) carried out on 7 January 1961. Plantagenet Bank is one of three flat-topped guyots of volcanic origin, on the same platform as Bermuda (Woollard, 1953), and rises from 2400 fathoms to 30 fathoms below sea level (Fig. 2). The survey track spacing was 0.5 mile, and positioning was provided by Loran-C with an estimated accuracy of + 1000 feet.

Figures 3, 4, and 5 are contour charts depicting the total intensity (F), inclination (1), and declination (D) of the magnetic field ob- served at 500 feet above sea level. The survey data were measured within a 4.5-hour period thus minimizing the effects due to tem- poral variations. The amplitude of the magnetic anomaly in the survey area is approximately 4200 eehias which is in agreement with an overall magnitude of 5000 gammas reported by Keller et al (1954).

The following procedure was used to compute the components of the magnetic anomaly over Plantagenet Bank: F, I, and D data in relatively undisturbed areas near Plantagenet Bank were extrapolated into the survey area. These data were then averaged to approximate the normal field values, (Fn = 51,280 gammas, In = 65.0°N, Dn = 13.8°W). A grid overlay with an interval of 0.5 mile was used for scaling F, I, and D values from Figures 3, 4, and 5. For each of 240 grid intersections, north (X), east (Y), and vertical (Z) components were computed. These directional components were then subtracted from their respective components of the normal magnetic field. The residuals represent

D

anomalous X, anomalous Y, and anomalous Z components of the magnetic field (Figs. 6, 7, and 8). The total intensity, inclination, and declination of the anomalous magnetic field are shown on Figures 9, 10, and 11, and were calculated from the anomalous X, Y, and Z components.

Several observations regarding the nature of the magnetic source may be made from an inspection of the anomalous components. Since the anomalous dip and declination represent the orientation of the lines of force referenced to the vertical direction and geographic north, respectively, the point at which the anomalous dip goes to 90° and the anomalous declination contours converge locates the dip pole in the plane of observation. The distortion of the anomalous declination and dip isolines in the northern portion of the survey area can be attributed to a localized secondary source. The direction of the anomalous X axis indicates the horizontal direction of magnetization. This is supported by the agreement between the computed direction of

magnetization (3399, see Sec. III) and the orientation of the

anomalous X axis (340°).

Nevill Gx) feast) Vey (2) i= Sl 2 eO~ Sze eS” ~ Cos 1S. k" Di Ones =o = | 20 * te5le SS? be Oak Gi A) a Smite Sy n} — > &— (are Fay SF eT te FLNOAE a AVS = 2S Srey ow A J ey ny Ey heat Tease Sale that LEP Rw re Al ey fe, 2) Sarai Ey anh: NO 2rd fg = S\ gwok siege = 46476 \ Yak = Cye AS Fe -| a 72b AME. 2 ma 5

TIL. ESTIMATES OF SOURCE BODY

Estimates of the probable shape, location, and magnetic properties of the source body were made with a modified computer technique of Vacquier (1962). Given a three dimensional, uniformly magnetized body and the observed anomaly, the program computes a direction and intensity of magnetization. A theoretical anomaly is then determined from the computed magnetization and the source body. The difference between the observed anomaly and the computed anomaly is a measure of the "goodness Oi wey"

For the Plantagenet feature, the anomalous vertical intensity was used as the input, and two source configurations were tested as possible solutions. The observed topography was discounted as being the source body, due to the localized areal extent of the magnetic anomaly; how-

ever, the slope characteristics of the bank were assumed to be a

reflection of the source body.

An east-west and nOBeRESouEn cross section through the center of the first model (run no. 1) is illustrated on Figure 12. The depth to the top of the model was based upon depth estimates from Stearn's method (Heiland, 1946), and Peters' method of tangents (1949). The slopes and base were arbitrarily chosen. This model resulted in a poor fit and excessive intensity of magnetization. The second model (run no. 2, Fig. 13) resulted in a better fit with a computed intensity of magnetization of 1200 gammas, and horizontal and vertical directions of 339° and 85°, respectively. From the results of run no. 2, it is evi-

dent that a body located between the model of Figure 13 and the ob- served topography would yield a closer fit. This is commensurate with the investigations of Officer et al (1952) which indicated that the

volcanics beneath the Bermuda Islands are close to the top. Further

computations were not made because additional geophysical and geological

information is required for a more significant solution.

Th

BERMUDA IS

URVEY AREA

FIGURE 1—SURVEY AREA

BERMUDA ISLAND

PLANTAGENET BANK

x z PS G © § 5

yrlal

From the proceedings of the Royal Society, A, Volume 222, p 371, 1954.

FIGURE 2—BERMUDA RISE (from photography of a model)

FLIGHT-LINE

FLIGHT-LIN pei"

Magnetic contours with flight-lines which provided Total Intensity data Contour Interval 200 Gammas

FIGURE 3—TOTAL INTENSITY (F) CONTOUR CHART—PLANTAGENET BANK

i , D 65°11’ |

aes ee FLIGHT-LINE RY

o°

FLIGHT-LINE

ie

ye

/;

B® vi

ae

\ ‘ \ Argus Island ©. . (Tower) 4 __FLIGHT-LINE :

FLIGHT-LIN

65° 1

Magnetic contours with flight-lines which provided Inclination data Contour Interval 0.5 Degrees

FIGURE 4—INCLINATION (1) CONTOUR CHART—PLANTAGENET BANK

FLIGHT-LINE

NY

Argus Island FLIGHT-LINE FLIGHT-LINE %

S. a Te ss ~>, Vo s Hi eh StSN Se = ‘0 0 , xe) |

Magnetic contours with flight-lines which provided Declination data Contour Interval 1 Degree

FIGURE 5—DECLINATION (D) CONTOUR CHART—PLANTAGENET BANK

Argus Island (Tower)

Contour Interval 200 Gammas

FIGURE 6—ANOMALOUS X CONTOUR CHART—PLANTAGENET BANK

10

07’

= my rae) = R 2) a |S a ~ =) FS 8

oF at. ae 00” *; % 594 58 574 Argus Island@

(Tower) 56’ sas § *K 31°] 55’ ? Hy | ]

T 14’ 13’ 12’ 65° 1 10 09’ 08’ 07’

Contour Interval 200 Gammas

FIGURE 7—ANOMALOUS Y CONTOUR CHART—PLANTAGENET BANK

11

Argus Island ©

(Tower)

Contour Interval 200 Gammas

FIGURE 8—ANOMALOUS Z CONTOUR CHART—PLANTAGENET BANK

12

Contour Interval 200 Gammas

FIGURE 9—ANOMALOUS F CONTOUR CHART—PLANTAGENET BANK

13

Contour Interval 5 Degrees

FIGURE 10O—ANOMALOUS | CONTOUR CHART—PLANTAGENET BANK

Argus Island oO}

(Tower,

Contour Interval 10 Degrees

FIGURE 11—ANOMALOUS D CONTOUR CHART—PLANTAGENET BANK

15

Z measured

(Z) GAMMAS

(shy ae! 8 Nautical SS

3 4 9 (0) 1 2 3 4 i 6 Y 9 0

FATHOMS

on—1600 Gammas

2) < = = x (o) iS)

14 | Sea level

PLANTAGENET BANK (N-S CROSS-SECTION)

FATHOMS

_ MODEL NO. 1

totensity of Magnetization — 1600 Gammas. S FIGURE 12—MODEL CALCULATION OF SOURCE BODY—RUN NO. 1

16

(Z) GAMMAS

FATHOMS

2) < = = <x (O} Ss

FATHOMS

14 ve) Sea level

PLANTAGENET BANK (N-S CROSS-SECTION)

Topographic relief

Intensity of Magnetization—1200 Gammas

FIGURE 13—MODEL CALCULATION OF SOURCE BODY—RUN NO. 2

17

REFERENCES

Heiland, C. A., Geophysical Exploration, Prentice-Hall, Inc., New York, 1946.

Keller, F., Jr., Meuschke, J. L. and Alldredge, L. R., Aeromagnetic surveys in the Aleutian, Marshall, and Bermuda

Islands, Transactions American Geophysical Union, 35, 558-572, 1954.

Officer, C. B., Ewing, M. and Wuenschel, P. C., Seismic refractions measurements in the Atlantic Ocean. Part 4: Bermuda, Bermuda Rise, and Nares Basin, Bull. of the Geologi-

cal Society of America, 63, 777-808, 1952.

Peters, L. J., The direct approach to magnetic interpretation and its practical application, Geophysics, 14, 290-320, 1949.

Schonstedt, E. O0., and Irons, H. R., NOL Vector airborne magnetometer

type 2A, Transactions American Geophysical Union, 36, P5=L, WSS

Vacquier, V., A machine method for computing the magnitude and the di- rection of magnetization of a uniformly magnetized body from its shape and a magnetic survey, results of work supported by the Office of Naval Research under contract Nonr 2216(05), 1962.

Woollard, G. P., Crustal structure beneath oceanic islands, Proceedings of the Royal Society A, 222, 361-386, 1954.

18

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